Skullcandy's Director of Electrical & Acoustical Engineering, Dr. Tetsuro Oishi Visits InnerFidelity!

Dr. Tetsuro Oishi
Tet is from Japan and came to the U.S. to attend U. Mass at Dartmouth for his advanced degrees in acoustics and electrical engineering. There he designed and fabricated various prototype transducers for underwater applications such as directional broadband piezoelectric transducers and arrays, acoustic motion sensors, and bioacoustic transducers. He also developed a water pollution detection system using the acoustic resonance spectroscopy technique. Wow, cool stuff.

Out of school he went to work for Bose's Automotive division modeling audio in cars, and then into the headphone division where he worked on noise canceling technologies. He also had a hand in the AE2 design.

Early in 2010, Skullcandy recruited him to lead its effort to improve the audio design and quality of its headphones. Yet another piece of hard evidence that Skullcandy intends to become a strong and well-founded maker of headphones.

The Hesh 2.0
The Skullcandy Hesh ($50) has been around for some time. The Hesh 2.0 ($59-$69 MSRP, likely available in April at Target and Best Buy) is a complete revision, and is Tet's first full effort at Skullcandy. This headphone is a medium-low cost circumaural, sealed headphone; designed for a big-bass sound; and no doubt a potentially very popular headphone among the action sports crowd looking to upgrade from junker headphones.

I put the Hesh 2.0s through the measurements routine with Tet looking on, and we spent a good bit of time discussing how to get a good fit and seal on the head. Over the last year I've developed some techniques for getting the job done, and I was heartened to hear that Tet uses some of the very same methods in his testing. (Sorry, I've got some competing publications measuring headphones, so I'm not going to detail these techniques.)

The Hesh 2.0 measured fairly flat to 1kHz with, what I consider, an appropriate roll-off to 4kHz, and then a reduction in overall treble energy about 5-10dB reduced beyond what I would consider flat.

Subsequent to our visit, I spent a good bit of time listening to these headphones, and comparing them to the Sony MDR-XB500 and Audio-Technica ATH-WS55. I felt the Hesh and WS55 easily bested the XB500, which sounded woolly and bloated in comparison. The WS55 emphasized the bass and the mid/upper treble a bit more, but the Hesh seemed more even and coherent. I think the choice of having good bass extention; keeping the bass properly related to the mids; and then rolling off the highs to get rid of crappy harshness from compressed files, and give the impression of big bass, is the way to go for this type of can. (Overwhelming bass is overwhelming.) I think the Hesh 2.0 is going to be the go-to recommendation for the hoodie wearing, bass-heads out there.

General Headphone Talk
Tet gave me some super help interpreting headphone measurement data (and we'll get to that on the next page), but we spent quite a bit of time just talking about headphones in general. Well ... that's not quite right, we talked about a wide variety of very particular aspects of headphones. Because I'm free to opine, and he's constrained by the nature of his job, I'll quickly rattle through a few of the topics we covered and what my take was:

  • Headphone mini-plugs - Should be slender and have enough "neck" on them to reach into the jack while the player has a protective cover on it. Plugs should not be straight as it tends to stress the jack more; and I believe plugs should not be 90-degree angles, as it tends to snag on things more easily. I like a 45-degree angle plug. I showed him the plugs on the V-Moda M-80 as an example of a great plug.
  • Freedom of spin in "lay flat" headphones - If the headphones are laying flat with cushions down on a table in front of you, you should be able to pick the headphones up and spin the earpieces towards your ears and slightly beyond. But there should be a limit in how far they can spin so as to provide a little more pressure of the pads behind the ears than in front of the ears.
  • Headphone cables should have plugs at both ends - Headphone cables should attach to the earpiece with a straight 3.5mm mini-plug, and without any weird mechanical attachments. This will allow the manufacturer to make one cable that fits all of its headphones, and alternate cables to turn the headphones into headsets for iPhones, Androids, and other phones easily. Even if a manufacturer doesn't provide those options, having a standard connector will allow the user to buy aftermarket cable to suit the customer's style of use.
  • Tangle-free cables - It's a hot topic among headphone makers at the moment, and I showed Tet the cable on the Philips Citiscape Downtown. It's a flat cable you can wad to your heart's content, and when you let it go, nine times out of ten it just "sproings" open completely tangle free. Then I showed him a prototype I got from a company with a flat, woven-cover cable the is almost impossible to handle without tangling.

Most interesting to me was the feeling that we were two hard-core headphone enthusiasts, yammering on about headphones at about the highest possible level one could imagine. It was great fun, and immensely satisfying.

The coolest thing of all, though, was getting Tet's run-down on headphone measurements.

RPGWiZaRD's picture

No but seriously, I really like that Skullcandy finally tries to improve the audio quality of their headphones and hired Tets, I have much respect for this person. I may even go out and buy a Hesh 2 haha despite my basshead needs is wonderfully covered in M-Audio Q40 headphones (A seriously overlooked headphone. One person commented as the bass beating Ultrasone PRO900 in quality with similar quantity and having way better mids and having less stringent and neutral highs, not bad for 3.5x less cost).

plin's picture

Thank you both for sharing your knowledge with us, very interesting indeed!
I am curious about the flat impedance curve in orthodynamic headphones. Does it mean that there is no internal drive resonances, in the whole of frequency spectrum?

Tyll Hertsens's picture
That's a very interesting question, one that I have been thinking about since it's related to why the impedance response remains apparently wholly resistive no matter what. I don't know the answer however. I'll report if/when I learn more.
John Grandberg's picture
...and for many young folks Skullcandy is likely the place. So increasing the quality of those entry level options can only be a good thing. Hiring Tet says a lot about their intentions - they really don't NEED to do anything to improve their sound, because people will buy no matter what. But they seem genuinely interested in stepping up their game. I'm impressed.
arnaud's picture

Tyll, you get to hang around cool people, this was a neat article. The first discussion about mass / air spring resonance is particularly interesting because we were discussing this on head-fi in regards to inconsistencies between different lcd3 responses. My take on this was it's not that simple to diagnose because I assume it depends on several variables:
1. if the baffled plate is sealed or not
2. If the headphone is closed or open back
2. If the earpead is impervious (leather) or porous type
3. If the diaphragm is suspended (electrodynamic) with very low mechanical resonance or a tensioned membrane (electrostats, orthos to some extent it would seem).

Anyhow, Tets discussion toward the end about the dt48 got me curious and I wish I can find time to play around with Vibro-Acoustics simulation tool to get some more insight about what matters and what doesn't... Thanks for sharing, the geek in me much appreciated ;).

hydrocarbon's picture

Good technical content there, thank you! It's always worthwhile to to talk shop with someone who's a genuine expert in his field.

Regarding the DT-48s, "tightly sealed" is an understatement. They are airtight like no other headphone (or industrial hearing protection headset) I've ever used. They're essentially suction cups, particularly right getting a haircut. In fact, I suspect that taking them off too abruptly could damage eardrums!

(Frankly, the reason I got them is because I like German-made metal things. The fact that they sound quite good to me with certain types of music is just a bonus; I was mainly concerned with their build quality, history and aesthetics.)

miceblue's picture

I am quite pleasantly surprised with Skullcandy's recent headphones. I owned the Hesh (1st gen) until they broke after 6 months of use. Perhaps I'll give the Hesh 2's another chance. The graphs look really impressive when compared against HeadRoom's Hesh (1st gen) measurements.

zobel's picture


Looks like most headphones you measure show a dip in sound pressure level at around 6 or 7KHz.

The wavelength of sound at 7KHz is 50mm. One half wavelength is 25mm.
The ear canal on average is 25mm. Could this cause out of phase cancellation from the reflection off the stiff microphone diaphragm?

As you know,the human ear isn't really the same as the test head's, since the eardrum, middle, and inner ear have different acoustic compliance and resonant frequency than the microphone at the end of the dummy ear canal.

Do headphones have to be made non-flat in response above 1KHz to correct for the resonances created by closing off (more or less depending on type of headphone) the ear canal with the driver?

I'm wondering what the high frequency portions of your charts show. I was wondering if Tet and you talked about about your raw data and the correction curve you have been using?

It kind of looks like a 10dB/octave roll off from 2 to 6 or 7KHz, combined with a steep 20dB gain from 6 or 7KHz to around 10KHz, followed by a roll off of about 25dB from 10KHz to 20KHz indicates a flat perceived frequency response.

Very interesting! Please keep talking to us about this stuff.

Tyll Hertsens's picture
Actually, the head is designed to have the same ear canal resonances as the average human head. Also, the acousic impedance/compliance of the microphone diaphragms at the end of the ear canal are designed to mimic the human ear. So the head and it's hearing system is designed to achieve very near the average human in as many was as possible. Not perfect of course, and it's only the average of humans, so won't be exactly like yours or mine.

Most of the spikes and valleys above 6-8kHz, or so, are not from the driver, but are the result of various resonances of the ear canal, concha cup, and size and shape of the enclosed volume between the driver and ear, and behind the driver.

Tet and I did talk about the HRTF compensation used, and I think he agreed that the Independant of Direction Curve was a good one to use.

I think the peak at 8-10kHz is problematic, but I do think it's likely optimal for there to be a slight bump there on the graphs I make. The leading edge of the 300Hz squarewave having some, but not too much, overshoot is related, and there should be a slight overshoot for an ideal percieved response ... still thinking about that one, though.

yuriv's picture

If the head could talk, he (she?) would complain about that. You can see it in the graphs of so many cans, and it's also in the raw measured response. So it's not a result of the ID HRTF compensation. It's really there, and it's probably the result of the shape of the dummy's ears. In fact, almost none of the IEM measurements show that 5.5 kHz null.

Perhaps the reflected sound arrives at the opening at an odd multiple of half a wavelength later after the direct sound. If you could post photos of the head simulator's ears next to a ruler, that would be instructive. So would headphone measurements with different-shaped ears.

There is so much disagreement about which headphones sound correct, and--average ears on the head simulator or not--this is a big factor. For example, I almost never hear a null between 5-6 kHz. After playing with Sinegen with many, many cans, I noticed that for me, the null happens just under 4 kHz. I never hear that with IEMs or loudspeakers.

Jazz Casual's picture

Not playing devil's advocate here but given the variables at play, is it possible that we're attaching too much weight to headphone measurements as a reliable predictor for what we're supposed to be hearing?

Tyll Hertsens's picture
I guess that depends on how much weight you're actually putting on it. I certainly don't believe you can tell exactly what kind of listening experience you'll have from measurements, but I do think one can tell, in gross terms, wether it's got too little or too much bass; whether the headphones are carefully designed so as to have an even response; and whether they'll be "bity" sounding. But only when these features are strong and readily apparent.

However, once it appears the headphones are within a certain range, you simply must do a listening test to tell what they really sound like --- and, of course, then matters of individual ear variations and taste come into play.

ultrabike's picture

Amen Tyll

RPGWiZaRD's picture

One often overlooked thing is that only a very small portion of people are experienced enough and so deeply involved in the hobby for starters to be able to translate and separate their own personal taste from "objective evaluation" of a headphone. Rarely people want a perfectly flat looking response in a frequency response graph but many people think that's what to strive for while disregarding your own personal taste of how it SHOULD sound according to your ears/preferences and therefore some people may say one headphone has recessed highs when it doesn't just because your preference wants a 3dB boost...

Jazz Casual's picture

I make a point of studying headphone frequency response measurements. I welcome an objective point of reference in such an inherently subjective hobby. Frequency response graphs can't plot the intangibles of what makes a headphone appealing or unpleasant to my ears, but they have helped me to make sense of what I'm hearing across the frequency spectrum. I find them useful for comparative purposes and when considering headphones that I can't audition prior to buying.

Tyll Hertsens's picture
Glad to be of service. :)

(Jots down yet another note to measure PS1000.)

Jazz Casual's picture

That would be great thanks Tyll. :)

The Monkey's picture
Let's get Tet to some meets!
Tyll Hertsens's picture
I'll tell him about the NY meet.
mrspeakers's picture

Thanks for another interesting post.

It'd be fun if Tet came to New York.